1. Field of the Invention
The present invention relates to a front wheel- and rear-wheel drive vehicle in which either the front wheels or the rear wheels are driven by an engine and the other thereof are driven by an electric motor.
2. Description of the Related Art
Conventionally, there is known a kind of vehicle in which an alternating-current (AC) motor is used as an electric motor and in which the electric motor is made operable over the entire range of vehicle speed through an inverter control (see Japanese Published Unexamined Patent Application No. 159502/1991).
In order to cover the entire range of the vehicle speed as in the above-described prior art, it is necessary to employ an electric motor having a large output. In addition, an inverter circuit becomes necessary, resulting in a higher cost.
By the way, if an arrangement is made to operate the electric motor only at the time of starting the vehicle in order to limit the purpose of using the electric motor to the assistance of the vehicle""s starting, it is possible to use an inexpensive direct-current (DC) brush motor such is used as a self-starting motor or the like. However, if the DC brush motor is used always at the time of starting the vehicle, the durability of the brush will become a problem.
In order to deal with this kind of disadvantage, the following arrangement may be considered. Namely, there is provided a switch which is operated when the driver has determined the road to be slippery, and the electric motor is operated when the vehicle starts in a condition in which the switch is switched on. Other wise, slipping of the wheels to be driven by the engine is detected based on a difference in revolution speeds between the front wheels and the rear wheels. The electric motor is operated when those wheels give rise to slipping at the time of starting the vehicle.
However, the former system is troublesome because the operation of the switch becomes necessary. In the latter system, on the other hand, when the vehicle starts on a road surface such as a gravel road or the like having a high coefficient of friction, the electric motor is also operated when the wheels to be driven by the engine spin and throw gravel so that there occurs a difference in revolution speeds between the front wheels and the rear wheels. In such a case, as soon as the gavel is thrown from under the wheels, the vehicle can start even if the electric motor is not operated at all. It follows that the electric motor is inefficiently operated.
In view of the above-described disadvantages, the present invention has an object of providing a front wheel- and rear-wheel drive vehicle in which the inefficient operation of the electric motor is prevented and in which the electric motor is automatically operated to assist the starting of the vehicle on a road surface having a low coefficient of friction.
In order to attain the above and other objects, the present invention is a front wheel- and rear wheel-drive vehicle in which either the front wheels or the rear wheels are driven by an engine and the other thereof are driven by an electric motor, wherein the electric motor is operated when the vehicle starts, the vehicle comprising: means for detecting a driving torque of wheels to be driven by the engine; and control means for controlling to prohibit the operation of the electric motor when the driving torque to be detected during start-up of the vehicle exceeds a predetermined value.
When the vehicle starts at above a certain amount of acceleration, a driving torque above a certain amount or degree is applied to the wheels to be driven by the engine. Even when the wheels slip at the time of starting the vehicle on a road surface having a high coefficient of friction, a relatively large driving load is applied to the wheels due to the friction between the wheels and the road surface. The driving torque on the wheels therefore is above a certain amount.
On the other hand, when the wheels slip at the time of starting the vehicle on a road surface having a low coefficient of friction (e.g., on a slippery snowy road or the like), the driving load will not operate so much on the wheels, with the result that the driving torque of the wheels becomes small.
Therefore, if the predetermined value is set to such a lower limit value of the driving torque as will be operated on the wheels to be driven by the engine when the vehicle starts only by the drive of the wheels to be driven by the engine at an acceleration above a predetermined level, the driving torque of the wheels becomes less than the predetermined value when the wheels slip at the time of starting the vehicle on a road of low coefficient of friction. Consequently, the electric motor is operated to assist the starting of the vehicle. When the vehicle starts without giving rise to slipping of the wheels or when the wheels slip on a road surface having a high coefficient of friction, the driving torque of the wheels exceeds the predetermined value. Consequently, the electric motor is not operated.
In this manner, the electric motor is operated only in a condition in which the effect of starting assistance is desired. The inefficient operation of the electric motor can therefore be prevented.